Special anti-terror courts sitting in secret to determine how long suspects should be detained without charge are now under active consideration, it emerged yesterday.

Home Office sources confirmed that ministers are considering making a French-style "security-cleared judge" responsible for assembling a pre-trial case against terrorist suspects, with in-camera access to sensitive intelligence evidence, including currently inadmissible phone-tap evidence.

The plan under consideration, which echoes elements of David Blunkett's proposal last year for secret anti-terrorist courts, could also involve the use of security-vetted "special advocates" as legal representatives of those detained. But they would not be able to disclose the nature of the evidence under which their clients were held before being charged.

The proposal puts flesh on the point outlined by Tony Blair last Friday, when he said that part of the new anti-terror package would include "a new court procedure which would allow a pretrial process". He said it would provide a way of meeting requests by the police and security services that detention before charge should be extended from the current 14 days up to three months.

It was also confirmed yesterday that the prime minister's plan to ban Hizb-ut-Tahrir and its successor organisation, al-Muhajiroun, the two Islamist extremist organisations with the highest profile in Britain, is likely to need primary legislation before it can be enforced, as neither group is officially considered a terrorist organisation.

Whitehall sources said that the current Terrorism 2000 Act only allows "terrorist" organisations to banned; for Hizb-ut-Tahrir to be proscribed, legislation to extend the definition to radical extremist groups as well will be required. It is not known at present how "extremist" will be defined, and whether it would catch groups such as the British National party.

The list of "unacceptable behaviours" published by the Home Office includes fomenting terrorism, advocating violence and expressing "extreme views that are in conflict with the UK's culture of tolerance".

In a BBC2 interview broadcast last night, President Per vez Musharraf of Pakistan called for Britain to ban both organisations, saying they had issued an edict calling for his assassination. Australia also announced an investigation into Hizb-ut-Tahrir.

The decision to adopt secret anti-terrorist courts will mark a huge departure from England's centuries old "adversarial" system of justice. In France, where an inquisitorial system is used, an examining magistrate hears evidence from witnesses and suspects, orders searches and authorises warrants, before deciding if there is a valid case.

The approach was recommended by Lord Newton's committee of privy councillors in December 2003 as an alternative to Belmarsh detentions in cases where a conventional trial was not possible.

He said the approach could deal with the risk that prosecution would lead to the need to disclose sensitive material, and the risk that intelligence-based evidence might be excluded because of the rules of admissibility.

Roger Smith, director of Justice, the all-party law reform group, said there were enormous dangers in using "preventitive detention" and in not disclosing the case against somebody held without charge. "How does a person refute the allegation against them if they do not know what that allegation is?" he asked.

The official reviewer of the government's anti-terrorist legislation, Lord Carlile QC, yesterday endorsed the Newton approach. But the Liberal Democrat peer warned that the current attempt to charge three prominent radical Islamists with treason was unlikely to proceed.

Senior figures from the Crown Prosecution Service are to meet Scotland Yard officers this week to see if it is possible to charge the three radicals with a range of offences, including solicitation to murder as well as treason. The Treason Act was last used to hang the wartime Nazi propagandist William Joyce, alias Lord Haw-Haw, in the 1940s.

It emerged last night that Omar Bakri Mohammed, the radical cleric who claimed that the British government and public were to blame for last month's London bombings, has left the UK for Lebanon.